Many cardiac patients have conditions which can periodically result in excessively fast or erratic heartbeats. If not treated promptly, ventricular fibrillation or certain ventricular tachycardias can result in a fatal outcome. If such tachycardias are promptly detected and treated, such as by electric shock defibrillation, the result of such an attack can often be minimized. Such treatment is normally needed within a few minutes of the onset of the condition to be effective. Therefore, it can be critical to accurately detect such a condition as soon as possible after its occurrence.
In hospitalized patients, the availability of detection equipment and trained medical staff provides a high degree of early detection and treatment. However, persons who are susceptible to such life threatening conditions cannot be hospitalized constantly. It is desirable to have a detection and treatment device which can be utilized by patients especially those that are ambulatory and are not in a hospital setting. Once such device is shown in U.S. Pat. No. 4,929,690. That device incorporates both a detection and treatment mechanism that can be utilized by a non-hospitalized patient. The utilization of such a patient-worn device permits the person susceptible to tachycardia to participate in a relatively normal lifestyle while wearing a device that is comfortable and effective in treating a potentially dangerous arrhythmia condition.
Systems for detection of life threatening tachycardia may utilize electrical sensors to process ECG waveforms and detect QRS signals such as shown in the U.S. Pat. No. 4,928,690. Morphology of QRS signals, or the patient's heart rate may be utilized to determine potentially dangerous conditions. In addition, the change of the patient's heart rate may also be monitored. The patient heart rate or the change of heart rate, coupled with monitoring for a change in QRS morphology, can trigger a preset level to indicate a treatable condition.
Patient cardiac condition can also be indicated by analysis of the QRS signals. These prior QRS analysis system include monitoring of frequency and time based components of the waveform. U.S. Pat. Nos. 4,422,459 and 4,458,691 address frequency components indicative of certain cardiac conditions. U.S. Pat. No. 4,458,691 specifically address the utilization of segmentation of ECG signals, particularly high frequency components with an adaptive filter.
Ventricular tachycardia and ventricular fibrillation are two heart rhythms that are treatable by an electrical shock properly applied to the body of the patient. Both of these conditions occur along with a detectable high heart rate in the patient. Utilization of a threshold heart rate will detect these two conditions in many cases and treatment can begin. Unfortunately, other conditions such as, for example, supraventricular tachycardia also have a high heart rate and these are not treatable by electric shock therapy. Therefore, utilizing a detection methodology which relics only on heart rate to institute treatment may cause treatment to be rendered under conditions where shock therapy may be inappropriate. Therefore, it is desirable to have a method and apparatus which could detect treatable conditions and discriminate in situations such as supraventricular tachycardias which do not require shock treatment. Because it is desired to have such detection systems in situations where human medical assistance may not be available in a timely manner, it is also desirable that such apparatus and method be readily adaptable to patient-worn devices for use by ambulatory patients. Ambulatory patient-worn devices inherently have additional problems in that the patient is generally unrestricted in his movement and detection electrodes may not be able to maintain continuous conductivity. Therefore, it is desirable that detection systems be adaptable to function in a multiple logic decision making topography.